Considerations of energy saving and reduction of pollution, above all in an urban environments, are leading motor vehicle manufacturers to equip their models with an automatic starting/stopping system, such as the system known by the name of Stop and Go (registered trade mark).
As recalled by VALEO EQUIPEMENTS ELECTRIQUES MOTEUR in patent application FR2875549, the vehicles are enabled to function according to the Stop and Go mode (registered trade mark) by means of a reversible electrical machine, or alternator-starter, which is coupled to the thermal engine.
A typical Stop and Go (registered trademark) situation is that of stoppage at a red light. When the vehicle stops at the light, the thermal engine is automatically stopped, then, when the light turns green, the engine is restarted by means of the alternator-starter, as a result of detection by the system that the driver has pressed the clutch pedal, or any other action which indicates the driver's wish to restart his vehicle.
The development of these systems now imposes new constraints on motor vehicle parts manufacturers relating to compliance with minimum voltage thresholds of the battery during the current requirement when the starter is started. Thus, in their specifications, the motor vehicle manufacturers define a voltage threshold which is habitually between 7 and 9 volts for a 12 V network, below which the battery voltage must not drop.
During the starting of the thermal engine, the voltage of the on-board network of the vehicle thus remains at a value which is sufficient to guarantee the expected functioning of the equipment, and not to cause the occupants inconvenience by disrupting the operation of a car radio, for example
Generally, on alternator-starter which is used in an engine, when the inverter is activated after the stoppage, full-wave control will allow the stator current to be established up to a maximum value which will depend only on the battery voltage and the resistance of the supply circuit (resistance of the phase windings, of the supply cable, internal resistance of the battery and of the switching elements), since the counter-electromotive force is negligible.
The current requirement, which can reach approximately 1000 A, creates a substantial voltage drop on the on-board electrical network, and requires power electronics of a size suitable for this high current.
In the patent application FR2843841, the company VALEO EQUIPEMENTS ELECTRIQUES MOTEUR proposes a device for control of an alternator-starter of a vehicle which can supply a voltage higher than that present on the on-board electrical network, such as to mitigate the voltage drop.
However, it will be appreciated that this solution does not solve the problem of the excess size of the power electronics, which is necessary only during the short transitory phase before the starter gets up to speed.